1. Field of the Invention
The present invention relates to a stereoscopic display method and apparatus for calculating a phase distribution of a hologram from 2-dimensional images at different visual point positions and displaying a solid image by an optical wave front conversion of a reproduction light and, more particularly, to a stereoscopic display method and apparatus for displaying a solid image as a moving image.
2. Description of the Related Art
Various kinds of studies and developments have conventionally been performed with respect to a display method of enabling a solid image to be stereoscopically seen. The conventional stereoscopic display method relates to the double-eye type represented by the glasses type and is based on a system for obtaining a stereoscopic feeling by a convergence of both eyes or a parallax of both eyes by allowing different images to be seen by the right and left eyes. There is a Lenticular method of the multieye type which is an extension of the double-eye type method. The principle for the stereoscopic observation of the Lenticular method is similar to that of the double-eye type. According to such conventional stereoscopic display methods, even when the observer moves the head to the right and left, no difference (kinetic parallax) occurs in a solid image to be observed, only an unnatural solid image can be seen.
A holographic stereogram can be mentioned as means for eliminating such a problem. In the holographic stereogram, a 2-dimensional video image including a parallax is recorded on elongated slit-shaped segment holograms which are long in the vertical direction and a number of such segment holograms are arranged in the horizontal direction. Therefore, even when the observer moves the head to the right and left, a natural stereoscopic feeling can be obtained. There is also a holographic stereogram including a parallax in the vertical direction.
The conventional holographic stereogram having a horizontal parallax will now be described. As shown in FIG. 1, objects 332 and 334 are photographed while changing the position of a camera from a position 330-1 to a position 330-n. As shown in FIG. 2, a laser beam 342 is irradiated to a film 336 obtained by the photographing operation in FIG. 1. The light transmitted through the film 336 is projected onto a diffusion plate 340 through a lens 338 and an object light 344 is irradiated onto a hologram dry plate 352. A slit 350 of a slit plate 348 is located in front of the hologram dry plate 352 in correspondence to the photographing position. An interference fringe (phase distribution) is exposed and recorded by an interference with a reference light 354.
As shown in FIG. 3, there is also a method of forming an image hologram. Namely, a laser beam (reproduction light) is irradiated onto the hologram dry plate 352 formed in FIG. 2 so as to be converged at a reproduction light source 355 shown as a virtual image. The laser beam is wave front converted into the object light 344 by the hologram dry plate 352. Another hologram dry plate is provided at the display position of the image by the object light 344. A reference light 362 is irradiated to the dry plate and the image on the dry plate is exposed, thereby forming an image hologram 360. As shown in FIG. 4, by irradiating a reproduction light 364 to the image hologram 360, it is converted into the wave front, so that a solid image can be recognized when it is seen from an eyesight region 366.
In the case where a moving image which changes with the elapse of time is stereoscopically displayed by such a conventional holographic stereogram, a phase distribution of the hologram dry plate 352 shown in FIG. 2 is obtained by a calculation, a display apparatus such as a liquid crystal device or the like is driven, the phase distribution is expressed, and the reproduction light is converted into the wave front.
In the case of displaying a solid image of a moving image by expressing the phase distribution obtained by the calculation on the display apparatus, however, there is a problem that a fatigue of the observer is large. This is because in the case of the moving image display, the position of the solid image to be displayed changes in accordance with the elapse of time. In the holographic stereogram, however, the position (hereinafter, referred to as "virtual screen position") at which a 2-dimensional image is formed is fixed to calculate a phase distribution of the hologram forming surface. Therefore, even when the position of the solid image changes with the elapse of time, the virtual screen position at which the 2-dimensional image is produced doesn't change, so that a fatigue of the observer increases. That is, an angle of convergence of the eyes of the observer is changed in accordance with the time-dependence position of the solid image. The image is actually fixed to the virtual screen position, however, so that when a focal point is set to the virtual screen, a contradiction occurs between the focal point and the angle of convergence and a disturbance occurs in the ecological feedback and the observer is tired.
Moreover, a fact that there is a tendency such that the allowable range of the positional relation between the screen and the solid image is narrowed as the solid image is located near the screen is known (Masuda, "3-dimensional Display", issued by Sangyo Tosho Co., Ltd., pages 42 to 44, May 25, 1990). There is, consequently, a problem such that when a moving image is displayed, the position of the solid image which is displayed on the virtual screen existing at a relatively near position is away from the screen, and a degree of fatigue of the observer further increases.